DE1904739A1 - Process for the aftertreatment of polyethylene in a shear unit - Google Patents

Description

Badische Anilin- & Soda-Fabrik AG

Our reference: O.Z. 26 006 Ms / AR

6700 Ludwigshafen, January 30, 1969 Process for post-treatment τοη polyethylene in a shear unit

The invention relates to a method for the aftertreatment of polyethylene in a shearing unit, in particular a screw press, the polyethylene produced in a high pressure process in molten form with the exclusion of atmospheric oxygen. Form at a temperature of 200 ° to 300 ⁰ C and a viscosity of 5 x 10 to 1 χ 10 ^v Poise is introduced into the shear assembly and exposed to a shear gradient.

For the production of semifinished products and standard parts from polyethylene melt or granules, it is preferred to use polymers which have good flow properties and good thermal stability ^{at the usual processing temperatures of over 200 0.} Semi-finished products made from these polymers, such as tubular films, pipes and plates, or finished parts such as bottles, household appliances and toys must have good mechanical strength, smooth surfaces that are as shiny as possible, and in some cases also good transparency. They should withstand stress corrosion cracking caused by atmospheric influences or by wetting agents (surfactants).

In the production of polyethylene, the different residence times of the reaction mixture in the reaction chamber and the subsequent separators as well as different polymerization temperatures in the individual sections of the polymerization plant, e.g. a tubular reactor, often result in proportions in the polyethylene that are in the molecular weight and in the molecular weight distribution and consequently in distinguish their rheological and also their optical properties from the main mass. In addition, these melts can contain proportions of crosslinked polyethylene particles, so-called specks. Molded parts that are made from such melts are usually unusable or have considerably reduced sales value.
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Attempts have been made again and again to fix these adhering to the polyethylene melt To remedy the deficiency already during the polymerization. So far, however, it has not yet been possible to carry out the high-pressure polyethylene process in such a way that all the properties of the polyethylene are optimal. The quality could only be partially improved by subsequent process steps.

For example, it is known to melt the polyethylene in To expose homogenizing units to a shear gradient, the disruptive components being crushed and homogeneous in the melt to be distributed or resolved. So far, attempts have been made to achieve high shear stresses in the shear areas of the homogenizing units to reach. For this purpose, the shear surfaces coming into contact with the polyethylene melt were cooled. Through this The energy consumption of the homogenizing units increased, but only as a result of the increase caused by the cooling Resistance in the gap. The drive energy was so in one very narrow area converted into heat and thus the actual effect is missed.

It is therefore preferred to homogenize polyethylene granules in order to carry out the homogenization at relatively low temperatures can. However, these granules carry adsorbed atmospheric oxygen into the homogenizing unit, the one with the polyolefin reacts and forms cross-links, but also malodorous degradation products. These components act as a further processing Lubricants, but also cause an increased effect on foils, for example Block.

It has also been suggested to get polyethylene from a separator a high-pressure polyethylene polymerization process to granulate or finished parts without further treatment. As practice shows, however, with this process the surfaces of the molded parts, especially extrudates, streaky and speckled, often rough and scaly.

According to another known method, attempts are made to produce granules for the production of standard parts with optimal properties,

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in particular without specks and nodules in the surface of the molded parts. To achieve cooling of the polyethylene melt to temperatures between 150-190 ⁰ C and intensive mixing at this temperature and subsequent extrusion and granulation. The melt should be stabilized at the same time. · ·

, It is disadvantageous that due to the low thermal conductivity of the Polyätbylenschmelze to cool the melt from the polymerization temperature in this process - is provided (250 ° 35O ⁰ C) to below 190 ⁰ C very large heat exchange surfaces or very long Yerweilzeiten must be respected. Layers of different viscosity and thus different flow properties are always formed, which have different dwell times. This also creates additional specks and streaks.

The processing of polyethylene melt also had a disadvantageous effect or -granulate by the previously known processes from ethylene contained in the melt. Moldings from not sufficiently degassed polyethylene often showed bubbles and foam spots.

It was therefore an object of the invention to provide a method for aftertreatment to develop of polyethylene that avoids the disadvantages described and provides a polyethylene that is used in the Further processing into semi-finished products and molded parts largely free of bubbles and inclusions from cross-linked components or degradation products and is thus improved optical, mechanical and rheological compared to the polyethylene used hitherto Has properties.

This task was achieved in the aftertreatment of polyethylene in a shear unit, the polyethylene produced in a high pressure process in molten form with the exclusion of atmospheric oxygen at a temperature of 200 ° to 300 ⁰ C and a Vis-

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viscosity of 5 x 10 to 1 χ 10 poise introduced into the shear unit and is exposed to a shear gradient, solved according to the invention in that the with the polyethylene melt in contact

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coming shear surfaces are heated to a temperature that equal to or higher than the temperature in the * seer-, aggregate entering polyethylene melt and that the ethylene melt with a shear rate of 1 χ 10 to

5-1
1 χ 10 see is sheared, with the proviso that the product

2 from the shear rate and dwell time in the shear area is 5 x to 5 x 10 ⁴ ".

Through these measures it is achieved that the polyethylene melt is only exposed to a relatively low shear gradient, the energy absorption of the, .Melt but relative through application high shear rates and correspondingly adapted residence times is at least 0.1 kWh per kg of melt. the P Heating of the shear surfaces coming into contact with the melt also causes the melt to open the shear area largely in the form of a plug flow and thus approximately the same dwell times for all those passing through the viewing area Partial flows are guaranteed. The distances between the shear surfaces delimiting the shear area or areas depend on the properties of the polyethylene used and the throughput. As a homogenizing unit, for example Screw presses are used that have one or more shear areas between the actual conveyor sections, which can be designed as rollers.

It has further been found that polyethylene at especially w high-quality semi-finished products or molded parts, ie solchen- can process with high gloss and good transparency with good mechanical properties when exposed to the shear rate Polyäthylenschmelze on entry into the shearing unit -Äthylen in an amount ranging from 0 , 01 to 0.5 wt. ^ Based on the total amount of the melt contains. Since the ethylene contained in the melt reacts with radicals contained in the melt or forming in the shear range, the reconstruction of undesirably long molecular chains or crosslinking of parts of the polyethylene melt is avoided and the polyethylene is given optimal mechanical and optical properties.

In addition, it was found that the after shear in

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the polyethylene melt still, with existing ethylene components the radicals of the melt continue to react if the access of atmospheric oxygen is still excluded. The reaction runs particularly evenly throughout the melt if heat is avoided from the melt. That can for example take place in that the walls of the container and pipelines as well as the shear surfaces in the Shear assembly to be heated to the temperature of the melt or above.

The gate part when using the method according to the invention compared to the methods previously used is in particular therein to see that the recycled polyethylene with improved processing properties better optical properties, a increased mechanical "strength and increased resistance to stress corrosion cracking. Further advantages of the Process according to the invention can be found in the following examples.

Example 1:

From the tubular reactor of a high pressure polyethylene polymerization is polyethylene melt density 0.918, melt index MI = 0.25 and with an A * thylengehalt of 0.12 wt. # At a! Temperature of 25O ⁰ O and a pressure of 4 atü is fed to a gear pump, which extrudes the melt without further preparation at a pressure of 100 atü from the ring nozzle of a film blowing machine to form tubular film with a wall thickness of 200 / U. Polyethylene processed according to this process produces tubular films with rough and flaky surfaces. The melt index of this tubular film is MI = 0.5, the shock resistance is 1200 cmkp / cm. After a few minutes, low molecular weight portions of the melt settle on the nozzle lips.

Example 2:

Polyethylene melt with the same physical and chemical properties As in example 1 from the same polymerization plant, under the same conditions as in example 1, a single spindle Screw press fed whose screw consists of two

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Conveyor sections and an intermediate distribution roller, the speed of which is 1250 seconds at 300 rpm in the shear area with a screw diameter of 120 mm and a distribution roller diameter of 117 mm, in the shear area of which the melt has a dwell time of 2 seconds this melt has a scatter value of 40 and a gloss of 85. The melt index is MI yes 0.15 and the shock resistance is 1800 cm kp / cm ² .

Example 3:

Polyethylene melt density 0.918, melt index MI = 0.25 and an ethylene content of 0.2 wt. # Is maintained at the 25O ° C by a screw extruder whose screw has Pörder- and shear areas at a temperature of 260 ⁰ G are conveyed through a ^{pipe heated to 250 0} C jacket with 100 mm inside width and 25 m length to a Polienblasanlage. With a throughput of 80 kg / hour. the dwell time in the pipeline is 2.23 hours and the flow rate is 11.1 m / hour. This results in a shear rate of 222 hours.

Tubular film extruded from this melt has a scatter value of 30, a gloss of 95, a melt index MI = 0.16 and a shock resistance of 2200 cm kp / cm.

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Claims (5)

- 7 - O.Z. 26 006 1 Q Π 4 7 3 Π Patent claims - ι 3 υ H / ο J 1. Process for post-treating polyethylene in a shear unit, in particular a screw press, the polyethylene produced in a high-pressure process - under exclusion of atmospheric oxygen in molten form at a Temperature from 200 to 300 ⁰ C and a viscosity of 5 x 10 ^ 5 * to 1 χ 10 is introduced into the shear unit and subjected to a shear gradient, characterized in that the shear surfaces coming into contact with the polyethylene melt are heated to a temperature which is equal to or higher than the temperature of the polyethylene melt entering the shear unit and that the polyethylene melt is sheared at a shear rate of 1 χ 10 ^ to 1 χ 10 ^ sec, with the proviso "that the product of shear 2 speed and dwell time in the shear area 5 x 10 bis 5 x 10 ⁴ . 2. The method according to claim 1, characterized in that the applied shear energy is at least 0.1 kWh per kg of melt. 3. The method according to claims 1 and 2, characterized in that the exposed to the shear gradient polyethylene melt on entry into the shear unit contains ethylene in an amount of 0.01 to 0.3 wt. ^ Based on the total amount of the melt. Badische Anilin- & Soda-Fabrik AG * Poise 009834/1788